1. Field of the Invention
In at least one aspect, the present invention relates generally to a method of making spray-formed articles and, more specifically, to a method of changing the shape of such spray-formed articles.
2. Background Art
The rapid production of prototype and production tooling is becoming increasingly important in the automotive industry. Most important is prototype and production tooling used in stamping, die casting and molding. Currently, small sets of prototype tooling can take from six to eight weeks to fabricate, while large prototype tools can take two to three months to produce. Large production dies can take up to a year to fabricate. Furthermore, prototype spray-form tooling has potential to produce production volumes because such prototypes can be made from steel which has sufficient durability and robustness. The spray-form process is capable of producing prototype tooling in less than one month at a cost that is equal or lower than conventional methods.
In the spray-forming process, hot molten steel droplets are sprayed onto a ceramic pattern (i.e., ceramic mold) to form a desired tool. The ceramic pattern is essentially the reverse of the desired tool to be produced. Typically, the ceramic pattern is produced using conventional casting techniques. A ceramic slurry is poured onto a master model and solidified to form the ceramic pattern. When solidification is complete, the resulting ceramic pattern is put through a series of heat cycles and becomes the receptor onto which metal is sprayed to form a deposit in the shape of the desired tool. The spray-forming process requires that such a ceramic pattern be made from a material that has excellent dimensional accuracy and superior surface finish. A suitable ceramic material is aluminum oxide.
One typical spray-forming process comprises wire-arc thermal spraying. In a common type of wire-arc spraying, electric current is carried by two electrically conductive, consumable wires with an electric arc forming between the wire tips. A high-velocity gas jet blowing from behind the consumable wire tips strips away the molten metal, which continuously forms as the wires are melted by the electric arc. The high-velocity gas jet breaks up or atomizes the molten metal into finer particles to create a distribution of molten metal droplets. The atomizing gas then accelerates the droplets away from the wire tips towards the ceramic pattern where the molten metal droplets impact the ceramic pattern to incrementally form a deposit in the shape of the desired article.
The desired article is then removed from the ceramic pattern. The removal is typically accomplished by chiseling off the majority of the ceramic pattern, removing residual ceramic from the surface of the desired article using a glass bead blaster, and then cutting off the perimeter of the metal deposit with a high pressure water jet. In the case of a tool, the completed tool is then mounted and used to produce parts in conventional stamping, die casting, molding, or other tool-usable processes.
Residual stresses are controlled in the spray-formed article by regulation of the metal deposition temperature. Such temperature control allows for good geometric accuracy. However, occasionally, unavoidable temperature deviations produces a tool that is slightly warped at one or more regions of the tool. Typically, in the spray-forming process, these warped regions are under compressive tension and bow concavely relative to the surface that was in direct contact with the ceramic mold.
Although the above process for making a spray-formed article has worked well, there is still a need to correct the warped regions in the occasionally produced spray-formed tool. Typically, such a deformed part will not be corrected and will instead be discarded. Correction of such deformed articles will reduce costs associated with scrap loses and further improve the throughput of the spray-formed process.
The present invention overcomes the problems encountered in the prior art by providing in one embodiment a method of correcting the shape a spray-formed article. The method of present invention comprises:
a) providing a spray-formed article having a working surface and a non-working surface wherein at least one portion of the spray-formed article has a concave bowing relative to the working surface;
b) applying sufficient heat to the non-working surface to permanently remove at least a partial amount of the concave bowing in the article;
c) measuring the deviation from the predetermined shape in the at least one portion of the article; and
d) repeating steps b) and c) until the at least one portion has reached a predetermined shape;
wherein the heat applied in step b) does not substantially mar the surface of the working surface. A number of sources may be used to accomplish the tension relief of the present invention. Preferred heat sources are spatially confined heat sources such as lasers, flames produced from torches, and radiant heat sources.